Obesity rate will reach at least 42%

Projections suggest obesity among U.S. adults may not plateau until 2050

November 4, 2010 |
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Jon Chase/Harvard Staff Photographer

“Our analysis suggests that while people have gotten better at gaining weight since 1971, they haven’t gotten any better at losing weight,” says lead author Alison L. Hill (right), who worked on the study with co-authors David G. Rand (left), Martin A. Nowak, and Nicholas A. Christakis.

Researchers at Harvard University say America’s obesity epidemic won’t plateau until at least 42 percent of adults are obese, an estimate derived by applying mathematical modeling to 40 years of Framingham Heart Study data.

Their work, published this week in the journal PLoS Computational Biology, runs counter to recent assertions by some experts that the obesity rate, which has been at 34 percent for the past five years, may have peaked. An additional 34 percent of American adults are overweight but not obese, according to the federal government’s Centers for Disease Control and Prevention.

The Harvard scientists say that their modeling shows that the proliferation of obesity among American adults in recent decades owes in large part to its accelerating spread via social networks.

The projections by Hill and colleagues are a best-case scenario, meaning that America’s obesity rate could rise above 42 percent of adults. One silver lining is that their model suggests the U.S. population may not reach this level for another 40 years, making the future rate of increase much more gradual than over the past 40 years. Only 14 percent of Framingham Heart Study participants were obese in 1971.

Hill, Rand, and colleagues found that a nonobese American adult has a 2 percent chance of becoming obese in any given year — a figure that has risen in recent decades — and that this number rises by 0.5 percentage points with each obese social contact, meaning that four obese contacts doubles the risk of becoming obese.

By comparison, an obese adult has a 4 percent chance of losing enough weight to fall back to merely “overweight” in any given year. This figure has remained essentially constant since 1971.

“These results suggest that social norms are changing the propensity for becoming obese by nonsocial mechanisms, and also magnifying the effect that obese individuals have on their nonobese contacts,” the scientists write in PLoS Computational Biology.

“Our analysis suggests that while people have gotten better at gaining weight since 1971, they haven’t gotten any better at losing weight,” says lead author Alison L. Hill (right), who worked on the study with co-authors David G. Rand (left), Martin A. Nowak, and Nicholas A. Christakis.

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